The most important starting material for molybdenum is molybdenite. It is usually found as a very low grade ore containing only a few tenths of a percent of molybdenum disulfide. The ores are processed by flotation, to a nearly pure molybdenum disulfide. The standard process for recovering molybdenum from the disulfide is by roasting in air in which the disulfide is oxidized to sulfur dioxide and the molybdenum is converted to molybdenum trioxide. The molybdenum trioxide is then sublimed to produce a pure grade material. Alternately, to produce molybdenum trioxide of lesser purity, the molybdenum trioxide can be acid washed to remove some impurities. After washing, the oxide can be reduced directly to molybdenum metal or further purified by being dissolved in ammonia, filtered to remove iron and insoluble hydroxides, then crystallized as pure ammonium paramolybdate or ammonium dimolybdate. The ammonium salts can be fired to molybdenum trioxide and finally reduced to the metal.
The main disadvantage of roasting is the production of sulfur dioxide which is one of the pollutants causing acid rain. Another disadvantage is that the temperature of roasting is critical since the molybdenum trioxide produced can either sublime or melt, both undesirable results. Another disadvantage is that the material must go through either a second firing to sublime the material for purification or through some type of acid washing. Both methods are time consuming and costly.
It would be desirable to recover molybdenum from molybdenum disulfide by a process which avoids these disadvantages.